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In a lecture on Monday, Mr Rowley said: "A deeply concerning characteristic is how both far-right and also Islamist terrorism are growing, allowing each side to reaffirm their grievances and justify their actions."

As well as five attacks that hit Britain last year, police and MI5 intervened to disrupt an unprecedented number of suspected plots.

Ten conspiracies of an Islamist nature have been stopped since the Westminster attack in March last year, Mr Rowley said.

He added that a further four extreme right-wing inspired plots were prevented.

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His remarks underline the authorities' mounting concern over the far-right threat, which was laid bare in the murder of Labour MP Jo Cox in June 2016 and the Finsbury Park attack a year later.

In December 2016, National Action - which Mr Rowley described as a "homegrown, white supremacist, neo-Nazi terrorist organisation"- became the first extreme right-wing group to be banned under terrorism laws.

MI5 has been given an increased role in the response to domestic extremism, including far-right threats, which police have historically led on.